1. Field of the Invention
The present invention relates, in general, to flame retardant oligomers, and particularly concerns a method of preparing flame retardant oligomeric phosphate esters.
2. The Prior Art
Oligomeric phosphate esters are known and are described in U.S. Pat. No. 3,767,732. U.S. Pat. No. 3,767,732 patent discloses compositions having the structural formula: ##STR2## wherein a stands for 1 or 0, n stands for a number between 0 and 4, R.sub.1 stands for at least one halogenated hydrocarbon radical and at least one hydroxylated radical of the structural formula: ##STR3## and R.sub.2 stands for a radical of the structural formula: ##STR4## The substituents R.sub.3 and R.sub.4 respectively stand for a hydrogen atom or a hydrocarbon radical having from 1 to 6 carbon atoms and being halogen substitued, if desired, and m stands for a number between 1 and 10.
The method of preparing the compositions of U.S. Pat. No. 3,767,732 comprises reacting a compound of the structural formula: ##STR5## wherein R.sub.5 is chloroethyl or 2,3-dibromopropyl and b is 0 or 1; in the presence of between about 0.1 and 2 percent of a phosphorus acid stabilizer and between about 0.1 and 2 percent of disodium phosphate with a polyphosphoric acid or a mixture thereof with P.sub.2 O.sub.5. A reaction mixture of partially esterified polyphosphoric acids is prepared by this step. The reaction mixture is then further reacted with an epoxide having the structural formula: ##STR6## wherein R.sub.3 and R.sub.4 are as defined above; until said partially esterified polyphosphoric acids are completely esterified. The product is a reactive flame retardant having hydroxyl numbers from about 100 to 300 milligrams of KOH per gram of product.
It is known that reactive flame retardants are undesirable for flexible polyurethane foams. They are detrimental to the flexibility of the foams and can cause other undesirable side effects to the foam properties.